There are many chemical applications, particularly analytical applications, involving the use of liquid solvents, reactants, or the like wherein the presence of dissolved gases, particularly air, is undesirable. An example of such an application relates to the mobile phase in high performance liquid chromatography where the presence of even small amounts of dissolved gases, and in particular oxygen, interferes with the accuracy and sensitivity of the results obtained. For example, air dissolved in the mobile phase can manifest itself in the form of bubbles, with the bubbles causing measurement noise and drift as the mobile phase passes through a detector. If the dissolved species be chemically active, as in the case of oxygen in air, unwanted changes or deterioration in the mobile phase can occur. Indeed, the detrimental effect of the dissolved species is related to the relative concentration of the species in the mobile phase. These undesirable species are typically removed by a known degassing process. It correspondingly follows that the more efficient the removal or degassing system is, the more accurate and desirable the system is.
Liquid degassing is necessary to many processes and, consequently, has long been actively pursued. Techniques for liquid degassing have included operations such as heating or boiling of the liquid to be degassed, exposing the material to a reduced pressure environment or vacuum, and use of combinations of heat and vacuum to reduce the amount of dissolved gases in the liquid. Exposure to ultrasonic energy has also been employed. As conventionally applied, however, these traditional techniques have generally fallen short of the desired degree of separation efficiency.
An additional means of degassing liquid involving the passing of a fine stream of bubbles of inert gas such as helium through the solution to be degassed is shown by Bakalyar et al (U.S. Pat. No. 4,133,767), and in apparatus such as is disclosed in Sims et al (U.S. Pat. No. 4,994,180) which was coinvented by the co-inventor in the present application and assigned to the same assignee as in the present invention.
Vacuum degassing through a membrane apparatus has long been known, and generally utilizes a length of relatively small diameter, thin-walled semi-permeable synthetic polymer resin material contained within an enclosed chamber and held under a reduced pressure or vacuum. To perform the degassing, the liquid to be degassed is caused to flow through the chamber. One such apparatus is shown by Sims (U.S. Pat. No. 5,340,384), which was co-invented by the co-inventor in the present application and assigned to the same assignee as in the present invention. Other such devices are shown in U.S. Pat. Nos. 5,183,486, 4,430,098, and 3,668,837.
While each of these devices employ a flow-through tube vacuum degassing approach, there remains a need, particularly with devices associated with high performance liquid chromatography (HPLC) instruments, to make degassing of liquids, and in particular the mobile phase, more efficient. One particular limitation or drawback associated with known devices concerns the efficiency of the degassification operation with respect to the composition of the tubing itself. Materials presently used in degassing applications include PTFE, PFA, and silicone rubber. Such materials, while generally suitable for degassing applications, require that tube wall thicknesses be as thin as possible due to the relatively low gas permeability of such materials.
Amorphous perfluorinated copolymers reportedly have substantially higher permeabilities for certain gases than the corresponding permeabilities of PTFE. The present inventors have found that by using amorphous perfluorinated copolymers, such as those marketed by Du Pont under the tradename Teflon AF, in a tubular configuration, increased permeability over similar PTFE tubing is achieved. Thus, greater gas mass transfer rates may be obtained through degassing tubes fabricated from Teflon AF such that Teflon AF degassing tubes may be fabricated with increased wall thicknesses while retaining desired degassing capabilities. Increased tube wall thicknesses permit the undertaking of applications requiring higher pressures.
Because of the enhanced gas permeability property of materials utilized in accordance with the present invention, the diffusion rate of atmospheric gases from the mobile phase being degassed through the tubing wall is significantly increased. It appears likely that the increased gas permeability enhances the function of free (void) volume in the polymer component.
Known degassing systems typically perform the degassing function on the mobile phases prior to the point where mobile phases enter a proportioning valve, where the mobile phases are proportioned into discrete volumes (slugs) for processing through the HPLC apparatus. Generally, each mobile phase is degassed while flowing through long tubes that connect each mobile phase source to the proportioning valve apparatus. Because of the low permeability of the materials commonly utilized in the degassing tubes, each degassing tube must be quite long to provide the necessary mobile phase residence time in the degassing tube to allow the entrained gas to escape from the mobile phase.
Such a method of degassing the mobile phases presents several disadvantages. The cumulative size of the degassing tubes limits the location of such tubes within the HPLC apparatus, and results in a relatively large space or volume being required to house the degassing system. Additionally, known degassing methods position the degassing tubes upstream from the proportioning valve apparatus due to the length of tubing required to sufficiently degas the mobile phase. In HPLC systems where multiple mobile phases are utilized simultaneously, the mobile phases are mixed into one stream in the proportioning valve apparatus. Even if the mobile phases being received in the proportioning valve apparatus are properly degassed, subsequent mixing may result in formation of gas bubbles in the mixed or blended mobile phase stream. Thus, degassing downstream from the proportioning valve apparatus (post-blending) is desired.
Accordingly, it is a principal object of the present invention to provide a more efficient flow-through vacuum degassing system using one or more tubes formed from an amorphous perfluorinated copolymer.
A further object of the present invention is to reduce the required inside diameter and length of the degassing tube.
A still further object of the present invention is the provision of a degassing tube having a single lumen.
A yet further object of the present invention is the provision of a degassing tube having multiple lumen.
Another object of the present invention is to provide a means for interconnecting liquid chromatography instrument components which interconnecting means simultaneously degases the mobile phase while in transit between the components.
A still further object of the present invention is the provision of a degassing tube which has an increased wall thickness while maintaining a desired level of permissivity, thereby allowing the degassing tube to withstand higher vacuum environments.
A yet further object of the present invention is to efficiently degas the mobile phase in a liquid chromatography system after multiple mobile phase streams have been blended and proportioned into an individual mobile phase stream.
A still further object of the present invention is to efficiently degas the mobile phase for delivery to a liquid chromatographic auto sampler.